Electrode for electronradiography imaging chamber

ABSTRACT

An imaging chamber for an x-ray system using a receptor sheet for a latent electrostatic image in the gap between electrodes. A chamber with substantially planar electrodes with each electrode having a spiral resistor and a low conductivity layer in contact with the resistor, with the spiral resistors interconnected by a third resistor across a power supply to produce electrostatic potentials at the gap surfaces the same as the electrostatic potentials of concentric spherical metal electrodes.

' United States Patent Azzarelli et al.

[ 1 Dec. 16, 1975 Proudian 250/315 A Attorney, Agent, or Firm-Harris, Kern, Wallen &

An imaging chamber for an x-ray system using a receptor sheet for a latent electrostatic image in the gap between electrodes. A chamber with substantially planar electrodes with each electrode having a spiral resistor and a low conductivity layer in contact with the resistor, with the spiral resistors interconnected by a third resistor across a power supply to produce electrostatic potentials at the gap surfaces the same as the electrostatic potentials of concentric spherical metal 6 Claims, 8 Drawing Figures 15 ELECTRODE FOR 3,859,529 1/1975 ELECTRONRADIOGRAPHY IMAGING CHAMBER Primary Examiner-Craig E. Church [75] Inventors: Teodoro Azzarelli, Los Angeles;

Eric P. Muntz, Pasadena; Paul B. ms ey Scott, Topanga, all of Calif. [73] Assignee: Xonics, 1nc., Van Nuys, Calif. [57] ABSTRACT [22] Filed: Nov. 8, 1974 [21] Appl. No.: 522,086

[52] US. Cl 250/315; 250/315 A [51] Int. Cl. G03B 41/16 [58] Field of Search 250/315, 315 A [56] References Cited electrodes UNITED STATES PATENTS 3,828,192 8/1974 Morsell 250/315 A 3 zayz/ /2 /3 I II/ I a 30 US. Patent Dec. 16, 1975 2 3,927,322

FIG. 1.

BACKGROUND OF THE INVENTION This invention relates to electronradiography and in particular, to a new and improved electrode construction for the imaging chamber. In electronradiography or ionography, an x-ray opaque fluid, typically Xenon gas, is used between two electrodes in an imaging chamber to produce a photoelectric current within the chamber, which current is collected on a dielectric sheet placed on one or the other of the electrodes, resulting in a latent electrostatic image. The latent image is then made visible by xerographic techniques.

An x-ray source is used to create primary photoelectrons in a gas in the gap between the electrodes of the imaging chamber. Typical imaging chambers have planar or cylindrical electrodes and the oblique incidence of the incoming x-ray produces geometric unsharpness in the resultant image. One solution to this problem is set out in the copending applications Ser. No. 388,212,

filed Aug. 14, 1973, entitled Ionography Imaging Chamber now US. Pat. No. 3,859,529 and Ser. No. 388,262, filed Aug. 14, I973, entitled Ionography Imaging Chamber for Variable Distance X'ray Source, both copending applications being assigned to the same assignee as the present application. In the imaging chamber of the copending applications, the electrodes are constructed in such a manner that the potential variations at the electrode surfaces correspond to that of concentric spherical equipotential in the imaging gap. In the preferred embodiment, a layer of finite and variable conductivity material is used for each electrode, with the potential applied between the center and periphery of the electrodes. Reference may be had to die copending applications for a complete discussion of the problem and the disclosed solution.

However, these copending applications call for con trol of the physical characteristics of a material, such as conductivity and/or thickness which must vary in a prescribed manner over a required range, all of which is difficult and complex. Accordingly, it is an object of the present invention to provide a new and improved imaging chamber electrode construction which is simple in design and inexpensive and easy to manufacture while achieving the desired electrostatic potentials at the gap.

SUMMARY OF THE INVENTION In the present invention, each electrode of the imag ing chamber includes a spiral resistor with a low conductivity layer or coating thereon. The spiral resistor may be in the form of a wire or an etched film or otherwise as desired. The spiral resistor preferably is provided on a circular flexible plastic substrate which can be folded over a square or rectangular plate of the shape desired for the radio graph. Other objects, advan tages, features and results of the invention will more fully appear in the course of the following description.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagramatic illustration of an x-ray system with an imaging chamber incorporating the presently preferred embodiment of the invention;

FIG. 2 is an electrical diagram of the imaging chamber of FIG. 1;

FIG. 3 is a diagram similar to that of FIG. Zshowing an alternative embodiment;

FIG. 4 illustrates an electrode of the imaging chamber of FIG. 1;

FIG. 5 is an enlarged partial view of the electrode of FIG. 4;

FIG. 6 illustrates the use of the electrode of FIG. 4 in a rectangular format;

FIG. 7 is a sectional view taken along line 7-7 of FIG. 6; and

FIG. 8 is a view similar to that of FIG. 5 showing an alternative form for the electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENT where the d) is the surface potential, V is the voltage drop across the gap at the center of the electrode (r=0), D is the distance from the electrode center to the x-ray source and d is the gap (distance between the electrodes). The distance r in a 14 X 17 inch chamber ranges as highasr,,= (l4/2) l7/2) 11 inches. D is typically inches. Thus r/D is small and and thus, to two percent accuracy for the above example It has been found that this potential distribution can be obtained by means of a spiral resistor at the electrode.

Consider a coil of resistor wire wound from the center out in a spiral, the wire having resistance p per unit length. A segment d0 is of length rdO and has a potential drop dd =l rdO due to a current I. Thus if the spiral pitch is constant (an Archimedian spiral), the turn to turn potential jump is ZTrIpr (4] or in the small spacing limit By setting we match equation (3), the required form of the surface potential for achieving the field of spherical electrodes. It is realized that the desired surface potential is not exactly obtained, as the wire turns are discrete and hence the potential tends to have discontinuous changes in slope.

The system as illustrated in FIG. 1 includes an x-ray source positioned for directing radiation to an object 11 which may rest on a table 12. An imaging chamber 13 carrying a dielectric receptor sheet 14 may be positioned below the table, with x-rays from the source passing through the object 11 and into the gas-filled gap 15 of the imaging chamber 13. The design of the imaging chamber itself is not a feature of the present invention and various of the known imaging chambers may be utilized.

The imaging chamber may comprise a housing with cover 21 and electrodes 22, 23 mounted therein, defining the gap 15 therebetween. Gas may be introduced into the chamber via line 28. A power supply 30 is connected to the center of electrode 23 via line 31, cover 21 and line 32, and is connected to the edge of electrode 22 via line 33. A resistor 34 is connected between the edge of electrode 23 and the center of electrode 22, providing a current path from the power supply through electrode 23, resistor 34, and electrode 22, as shown in FIG. 2. An alternative embodiment corresponding to that of the aforesaid copending application Ser. No. 388,262 is shown in FIG. 3, with a resistor 37 connected across the electrode 23 and/or a resistor 38 connected across the electrode 22. The resistors 37, 38 are used to compensate for changes in the distance between the .r-ray source 10 and the imaging chamber 13, in the manner described in detail in the aforesaid copending application.

One embodiment of the electrode 22 is shown in FIGS. 4, 5 and 6, with the electrode 23 having the same construction. A resistor wire 40 is placed on a substrate 41, typically a dielectric plastic sheet such as Mylar or Kapton. The wire 40 may be held to the sheet 41 by an adhesive, such as by applying an adhesive coating to the substrate and then positioning the wire thereon in the spiral configuration. A film or layer 42 of a lowconducting material is applied over the wire to provide a radial current path between the turns of the spiral. This layer may also serve as a protective layer for the resistor wire.

The layer 42 should be of low conductivity so that most of the current flows through the resistor wire, not through the layer radially from one wire turn to another. By using a spiral with 1 mm pitch, and assuming r= 28 cm. V /d= 10,000 volts/cm and D 1.5 meters, we find d/dr E 10 volts/cm at maximum, or AV 100 volts between the outermost winding. Allowing 1 ma of current flow, this calls for a surface resistance of 2 X 10 ohms per square for the low conductivity layer 42. By using a 0.1 mm carbon fiber for the resistor wire 40, which would have a lineal resistance of some 30 ohms/cm. the spiral for a 14 X 17 inch chamber would have a total resistance of a megohm and be driven by lO-2O milliamps of current. These figures are given by way of example so that one skilled in the art can select appropriate values for any specific embodiment, and are not intended as limitations.

Most x-ray equipment is designed to provide a rectangular picture, while the electrodes of the present invention are inherently circular. A rectangular or square electrode can be provided by utilizing a flexible material as the substrate and mounting the electrode on a plate of the desired size, with the extending edges of the electrode folded under and bonded to the reverse side of the plate. Referring to FIG. 6, the electrode 22 is mounted on a rectangular plate 45, with the electrode edges 46 folded over the edges of the plate. An opening 47 may be provided in the center of the plate 45 for the electrical connection to the inner end of the electrode resistor. For the 14 X 17 inch chamber previously mentioned, the spiral electrode should be at least 22 inches outside diameter, leaving substantial sections of the electrode for bonding to the underside of the plate.

An alternative embodiment for the electrode 22 is shown in FIG. 8, with a metal film resistor 50 carried on a dielectric plastic substrate 51 and having a low-conductivity layer 52 thereover. Typically this electrode may be produced by starting with a metallized plastic film such as aluminized Mylar and etching a spiral pattern to leave a metal film spiral resistor. By way of example, one could use a 0.1 micron thick aluminized coating and etch 0.1 mm spaces, leaving a spiral with a pitch in the order of 3 to 4 line pairs per millimeter. The etching can be carried out using known photoresist etching techniques. The aluminum film is sufficiently thin to serve as the spiral resistor. In an alternative embodiment, a thick film of resistance material may be utilized to achieve the desired resistance for the etched spiral resistor.

An advantage of the spiral resistor electrode is that only one voltage and two electrical connections per electrode are required. One connection is made to the outer end of the spiral resistor and the other connection is made to the inner end or center of the spiral resistor. The connection at the inner end may be made through the substrate, as by providing one or more small holes and utilizing a conducting plastic paste or epoxy in the holes in contact with the spiral resistor on one side of the substrate and a wire lead on the other side.

We claim:

1. In an imaging chamber for an .r-ray system, the combination of:

first and second substantially planar electrodes, each of said electrodes including a spiral resistor and a low conductivity layer in contact with said spiral resistor; means for mounting sid electrodes in the chamber in spaced relation defining a gap therebetween;

means for connecting a power supply to an end of said first electrode spiral resistor and to an end of said second electrode spiral resistor; and

a third resistor connected between the other ends of said spiral resistors completing a current path across the power supply to produce an electrostatic field in the gap with electrostatic potentials along the gap surfaces corresponding to the electrostatic potentials for concentric spherical metal electrodes so that extensions of the electric field lines in said gap converge substantially to a point.

2. an imaging chamber as defined in claim 1 includ ing a fourth resistor connected across the ends of said 5. An imaging chamber as defined in claim 1 wherein each of said electrodes includes a dielectric substrate with a spiral resistor film thereon and a low conductivity plastic layer over the film.

6. An imaging chamber as defined in claim 1 wherein each of said electrodes includes a flexible sheet with the spiral resistor thereon, and a plate carrying the sheet on one side thereof, with projecting sheet edges folded over onto the opposite side of the plate. 

1. In an imaging chamber for an x-ray system, the combination of: first and second substantially planar electrodes, each of said electrodes including a spiral resistor and a low conductivity layer in contact with said spiral resistor; means for mounting sid electrodes in the chamber in spaced relation defining a gap therebetween; means for connecting a power supply to an end of said first electrode spiral resistor and to an end of said second electrode spiral resistor; and a third resistor connected between the other ends of said spiral resistors completing a current path across the power supply to produce an electrostatic field in the gap with electrostatic potentials along the gap surfaces corresponding to the electrostatic potentials for concentric spherical metal electrodes so that extensions of the electric field lines in said gap converge substantially to a point.
 2. an imaging chamber as defined in claim 1 including a fourth resistor connected across the ends of said first electrode spiral resistor.
 3. An imaging chamber as defined in claim 2 including a fifth resistor connected across the ends of said second electrode spiral resistor.
 4. An imaging chamber as defined in claim 1 wherein each of said electrodes includes a dielectric substrate with a spiral resistor wire thereon and a low conductivity plastic layer over the wire.
 5. An imaging chamber as defined in claim 1 wherein each of said electrodes includes a dielectric substrate with a spiral resistor film thereon and a low conductivity plastic layer over the film.
 6. An imaging chamber as defined in claim 1 wherein each of said electrodes includes a flexible sheet with the spiral resistor thereon, and a plate carrying the sheet on one side thereof, with projecting sheet edges folded over onto the opposite side of the plate. 